Spacecraft have been developed that use low-thrust thrusters mounted to the spacecraft, such as, for example electric thrusters, that may be used for station keeping maneuvers. Such low-thrust thrusters can create significant disturbance torques when used for long duration burns, such as burns requiring thrust over a duration of three hours to seven hours.
In a typical communications satellite mission in which the satellite is required to maintain its attitude in a single orientation with respect to the earth, and is not required to perform frequent attitude reorientation maneuvers, a momentum biased spacecraft is the most economical configuration for attitude control. A significant advantage of a momentum biased spacecraft is that it requires minimal attitude control system hardware: a two-axis earth sensor, a pitch momentum wheel, and associated electronics. Specifically, it does not require an attitude sensor to measure attitude about the satellite-to-Earth nadir line, an axis herein called "yaw." To improve transient behavior, some designs add an ability to steer the spacecraft pitch momentum vector (e.g., using a V-wheel, or a single or double gimbaled momentum wheel), although this is not absolutely essential for adequate performance.
The use of ion propulsion systems for satellites has been considered for a number of years. See, e.g., Krulle, G., Zeyfang, E., "Combined Orbit and Attitude Control of Geostationary Satellites Using Electric Propulsion," IFAC Automatic Control in Space, Noordwijkhout, The Netherlands, 1982 and Marsh, Elbert L., "Attitude Control of Solar Electric Spacecraft by Thruster Gimbaling," Paper AIAA 73-1116, 10.sup.th Electric Propulsion Conference, Lake Tahoe, Nev., Oct. 31-Nov. 2, 1973. These early papers directed to the use of ion propulsion are survey papers postulating what might be feasible with ion propulsion. Early on in the implementation of the use of ion propulsion, it was recognized that gimbaled thrusters would be required in order to point the thrust vector of an ion propulsion engine through the spacecraft center of mass, or to intentionally offset the thrust vector to create desired torques for attitude control.
As ion propulsion designs moved closer to realization, attitude control system designers were forced to design control systems that were capable of sensing and correcting relatively large disturbances created by ion engines. It was generally concluded that, in addition to ion thrust vector steering, a yaw sensor would be required for the purpose of measuring yaw response to the disturbance torque created by an ion propulsion system. An article authored by T. G. Duhamel entitled "Implementation of Electric Propulsion for North-South Stationkeeping on Eurostar Spacecraft," paper AIAA 89-2274, AIAA/ASEM/SAE/ASEE 25.sup.th Joint Propulsion Conference, 1989, described a typical concept for attitude control on a momentum biased spacecraft using ion propulsion. The attitude control system design described in the Duhamel AIAA article was developed for the Eurostar spacecraft, and indicates that a yaw sensor must be added to measure yaw motion created by the ion engines. In the Eurostar design, a star sensor was added, even though the attitude control system already had a yaw gyro. The yaw gyro was not used to measure yaw motion created by the ion thrusters, because the yaw gyro was not capable of the numerous on/off cycles nor the long life operation necessary to support ion propulsion.
The following references describe several other attitude control system designs that have been modified to adapt a momentum biased spacecraft to use ion propulsion:
Nakashima, A., Fujiwara, Y., Okada, K., Yamada, K., Miyazaki, H., Matsue, T., "The Attitude Control Subsystem and Inter Orbit Pointing Subsystem for Communications and Broadcasting Engineering Test Satellite", 13 IFAC Symposium Automatic Control in Aerospace--Aerospace Control '94, Sep. 12-16, 1994;
Potti, J., Mora, E. J., Pasetti, A., "An Autonomous Stationkeeping System for Future Geostationary Telecommunication Satellites (An ARTEMIS based ASK System)," International Astronautical Federation, 1993;
Duhamel, T. G., Benoit, A., "New AOCS Concepts for ARTEMIS and DRS," Proc. Pirst International Conference on Spacecraft Guidance, Navigation and Control Systems, ESTEV, Noordwijk, The Netherlands, Jun. 4-7, 1991;
Mazzini, L., Ritorto, A., Astin, E., Attitude Control Design Concepts in the DTRM Satellites," i Pro., First International Conference on Spacecraft Guidance, Navigation and Control Systems, ESTEV, Noordwijk, The Netherlands, Jun. 4-7, 1991;
Duhamel, T. G., "Implementation of Electric propulsion for north-south Stationkeeping on the EUROSTAR Spacecraft," Paper AIAA 89-2274, AIAA/ASME/SAE/ASEE 25.sup.th Joint propulsion Conference, 1989; and
U.S. Pat. No. 5,349,532, "Spacecraft Attitude Control and Momentum Unloading Using Gimbaled and Throttled Thrusters," issued to Tilley, Scott W., Liu, Tung Y., Highman, John S., Sep. 20, 1994.
In each of the six references listed above, the modifications to the attitude control systems have included the addition of a yaw sensor to measure yaw attitude during the period when ion propulsion is activated. The yaw sensor may take the form of a sun sensor, a long life gyro, or a star sensor. In addition, the modifications to the attitude control system typically include some means for steering the ion propulsion thrust vector, such as a two-axis gimbaled mechanism, a translational mechanism, or a throttling mechanism.
The foregoing references provide a sampling of the state of the art in attitude control for spacecraft using ion engines in order to account for the relatively large disturbances created by firing such ion engines. The design for the ARTEMIS satellite also employed a star sensor used solely for attitude sensing during ion propulsion operation, as set forth in the above-noted Potti et al., Duhamel et al., and Mazzini et al. references. The ARTEMIS spacecraft was eventually launched circa 1994, but the ion propulsion payload failed almost immediately. In a totally independent design, Tilley et al., U.S. Pat. No. 5,349,532 discloses a control system that also incorporates a yaw sensor in the preferred embodiment. The control system disclosed in the Tilley et al. '532 patent employs both a sun sensor and a yaw gyro, either of which may be selected by a ground commandable switch. This suggests that Tilley et al. also believed that a yaw sensor was mandatory and that the life of a yaw gyro was insufficient to permit its use as the only option for sensing yaw rate.
A paper entitled "On Orbit Robust Control Experiment of Flexible Spacecraft ETS-VI", authored by Kida et al. and published in the AIAA Journal of Guidance, Control and Dynamics, Volume 20 No. 5 September-October 1997, described an attitude control systems used on ETS-VI, a Japanese satellite which flew an experimental ion propulsion payload. The basic attitude control system designed for ETS-VI is a zero-momentum, 4-reaction wheel configuration which also incorporates a yaw attitude sensor.
A paper authored by Nakashima et al. entitled "The Attitude Control Subsystem and Inter Orbit Pointing Subsystem for Communications and Broadcasting Engineering Test Satellite," published in 13 IFAC Symposium Automatic Control in Aerospace-Aerospace Control '94, September, 1994, describes engineering test satellite COMETS, that includes ion propulsion. The attitude control system on the COMETS satellite is a momentum biased design using a V-wheel concept to achieve three-axis torquing capability. The COMETS spacecraft also includes strapdown gyros to perform three-axis attitude determination, presumably to facilitate attitude control during ion engine firing.
Barsky et al. U.S. Pat. No. 5,765,780 entitled "Systematic Vectored Thrust Calibration for Satellite Momentum Control," is the only work known to the inventors of the present invention that attempts ion engine control for a momentum biased satellite without the addition of a separate yaw sensor. The Barsky et al. '780 patent employs an elaborate ground processing system to model and estimate the time-varying location of the spacecraft center of mass so that the parameters defining gimbal angles of the ion engine positioning system can be uplinked to the spacecraft on subsequent maneuvers. The Barsky et al. '780 patent discloses the use of ion thrusters mounted on gimbals. However, the gimbals are preset prior to each maneuver and not moved during the maneuver. The approach described in the Barsky et al. '780 patent was never flown on a spacecraft, because of the development schedule for the ion engines themselves. However, had this control system been used on a spacecraft, it would likely have experienced significant yaw pointing errors that were not foreseen. Such unforeseen errors arise, in part, from spacecraft center of mass motion created by movement of bi-propellant liquids within tanks mounted to the spacecraft. In the performance predictions used for the system disclosed in the Barsky et al. '780 patent, it was assumed that surface tension forces would be sufficient to restrain the motion of the bi-propellant liquids, when subjected to the micro-g forces induced by firing the ion engine. However, subsequent flight experience has shown this to be an invalid assumption. Accordingly, the design approach used in accordance with the present invention includes on-board, real-time, closed loop control and is much more robust to unmodeled disturbances than was the design in the Barsky et al. '780 patent.
Chan et al. U.S. Pat. No. 4,767,084, entitled "Autonomous Stationkeeping for Three-Axis Stabilized Spacecraft," discloses the concept of dumping pitch momentum during an east-west stationkeeping maneuver using bi-propellant thrusters. The control systems in accordance with the present invention includes a similar approach using gimbaled ion thrusters for dumping three-axis momentum during north-south maneuvers. However, momentum dumping, in and of itself, is not considered to be novel.
Chan U.S. Pat. No. 4,537,375, entitled "Method and Apparatus for Thruster Transient Control," discloses the concept of storing estimated values of disturbance torques during a bi-propellant stationkeeping maneuver and reusing the disturbance torque estimates for initialization at the start of the next maneuver. A similar initialization technique is used in accordance with the present invention.
The following references disclose the concept of disturbance torque estimation on momentum biased satellites:
Rahn, C. D., "Asymptotic Disturbance Rejection for Momentum Bias Spacecraft", AIAA Journal of Guidance, Control and Dynamics, September-October 1992; PA1 Beach, S. W., "Autonomous Compensation for Orbital Disturbances of Known Frequency", AAS-026, AAS Guidance and Control Conference 1983, Volume 51; PA1 Passerson, L., Bozzo, C. "Overcoming Unobservability in Three-Axis Stabilization of Satellites", Presented at the Guidance and Control Panel 37.sup.th Symposium, Florence, Italy, Sep. 27-30, 1983; PA1 Lebsock, K. L. "Magnetic Desaturation of a Momentum Bias System", AIAA Journal of Guidance & Control, Vol. 6, No. 6, November-December 1983; and PA1 Broquet, J., "Selection and Adaptation of a Control Law for a Double Gimbaled Momentum Wheel System on a Large Solar Array Satellite", Proceedings of the IFAC 6.sup.th World Congress, Boston Mass. Aug. 24-30, 1975. Such disturbance torque estimation is one of several features used in the present invention. In the above-mentioned references that disclose disturbance torque estimation, emphasis is generally placed on estimating roll/yaw torques for the purpose of improving yaw pointing performance during normal mode operations. In contrast, the control system forming part of the present invention places emphasis on estimating pitch disturbances, as accurate estimation of pitch disturbances is critical to achieving yaw pointing performance during operation of the ion propulsion system. PA1 1. The initial value of the roll and pitch disturbance torque estimates are set to the values determined after settling of turn-on transients of the previous maneuver using the same ion thrusters (on the first maneuver these values are initialized to zero); PA1 2. Feedback gains for attitude estimation are adjusted to avoid perturbations of the yaw estimate due to the turn-on transients. The yaw estimate is propagated open loop but is not substantially updated using the roll measurements; PA1 3. A yaw feed forward torque is computed based on the known pitch/yaw geometry of the ion engine and the sensed pitch disturbances. This yaw feed forward torque is negated and fed into the attitude control system control actuator (i.e., the momentum wheel), causing it to oppose the yaw torque arising from the thruster turn-on transient; PA1 4. The ion engine gimbals are steered to null the estimated disturbance torque (or to create a desired torque to dump momentum, if desired); PA1 5. If the momentum dump torque is non-zero, opposing torques are fed forward to the main attitude control system actuator (i.e., the momentum wheel) thereby effecting the desired momentum dump; and PA1 6. At the end of Phase 1 or early in Phase 2, the pitch and roll disturbance torque estimates are stored in memory for use the next time the same combination of ion thrusters are fired. PA1 1. Feedback gains in the attitude estimator are adjusted to capitalize on the roll/yaw coupling, in order to estimate yaw attitude. This roll/yaw coupling is weak and the estimation process is necessarily slow to avoid excessive noise from the attitude sensor; PA1 2. Feedback gains are also adjusted to ensure that the estimates of roll and pitch disturbance torques continue to track the time-varying disturbances from the ion engine; PA1 3. The ion engine gimbals are steered to null the disturbance torque (or to create a desired torque to dump momentum if desired); PA1 4. If the momentum dump torque is non-zero, opposing torques are fed forward to the main attitude control system actuator (i.e., the momentum wheel) thereby effecting the desired momentum dump.